You are browsing the Symfony 4 documentation, which changes
significantly from Symfony 3.x. If your app doesn't use Symfony 4 yet, browse the
Symfony 3.4 documentation.

Symfony Components

The Components implement common features needed to develop
websites. They are the foundation of the Symfony full-stack
framework, but they can also be used standalone even if you
don't use the framework as they don't have any mandatory
dependencies.